This invention relates generally to a brushless DC motor having a cylindrical air gap between rotor and stator, and especially designed for use for driving a rotating hard disc in a hard disc drive.
In permanent magnet motors, interfering or parasitic torques may develop because of the interaction between the edges of the magnetized poles and the slots of the stator. In small size motors, such as are used in disc drives, these problems can be even greater. The radial thinness of the rotor magnet is required by the limited diameter of the motor itself In such a small, high power motor, the air gap between the facing surfaces of the stator and rotor must also be minimized. The combination of high induction, the slotted stator, the minimal air gap, and the thinness of all the elements can easily lead to so-called torque unevenness or torque ripple.
The resulting running torque ripple in the motor is a source of audible noise. Since the disc drive is used in computers, such audible noise must be reduced in order for the product to be commercially successful.
It has been recognized that the reduction of torque ripple is an important goal. One current method known of reducing torque ripple is designing the spindle motor to have a trapezoidal back EMF (BEMF) wave form. Current methods used to achieve such trapezoidal BEMF include weak magnetization of the magnet pole center, or selection of stator slot/number/magnetic pole member. For example, it is disclosed in European patent 291,219, U.S. Pat. No. 8,847,712 Issued Jul. 11, 1989, that the number of stator poles should be kept approximately the same as the number of rotor poles. This is also taught to reduce torque fluctuations during starting or idling. However, in general according to the reference, this method requires at least nine wound coils per motor.
A problem with this approach is that in most known spindle motor configurations, other design considerations dictate the number of poles and slots to be used and the numbers are usually different. The selection of the number and arrangement of the poles and slots in most disc drive spindle motors is dictated by other considerations than the shape of the back EMF wave form.
Thus, the problem remains of providing a motor design which minimizes running torque ripple while remaining consistent with other design considerations for an efficiently operating disc drive spindle motor.
Another objective of the invention is to provide a motor design with by reduced running torque ripple, even though the number of slots may be dramatically different.
Therefore it is a primary general objective of the present invention to provide a permanent magnet excited motor having reduced running torque ripple in order to diminish the audible noise generated by the motor.
It is a further objective of the invention to provide a motor especially for use as a spindle motor in a disc drive which has a design providing reduced running torque ripple while remaining consistent with other design objectives of a disc drive spindle motor.
A further objective of the invention is to provide reduced running torque ripple by shaping and measuring the back EMF ripple of the motor as providing an accurate representation of the running torque ripple.
These and other objectives of the present invention are achieved by shaping the magnetization wave form created by the interaction between the rotating magnet of an outer rotor magnet supporting hub rotating past a slotted stator normally energized in the normal fashion. The magnetization wave form is shaped to provide a back ENT shape with a substantially flat top when a two phase wave form for a standard three phase motor is studied. It has been recognized that in this type of motor, this back EMF wave form shape will minimize torque ripple. The flat peak is achieved by adding shoulders to a standard single phase wave form, by providing a ten to fifteen percent null zone between each magnetic pole. This may be achieved alternatively by providing a solid magnet which is magnetized to establish a null zone; by molding the magnet in essentially a cylindrical shape with teeth, each tooth being aligned to create a magnetic pole during magnetization, with the air gap between the teeth creating the desired transition zone, or by scalloping the magnets inner diameter at the desired pole transition zones.
Other details and advantages of the present invention will become apparent to a person of skill in this field who studies the following figures in conjunction with the description of a preferred embodiment or embodiments.